The correct answer is (C), not (D). Let's take a look at both of them.
(D) says, "unable to sleep as much as usual"
From the passage, we know that d-fenfluramine will help to suppress an individual's carbohydrate cravings (lines 45-49). While consuming lots of carbohydrates when said person isn't craving carbs would likely make a person sleepy (lines 59-61), we don't have any evidence or reason to believe that simply taking d-fenfluramine will cause a person not to be able to sleep as much. Thus, (D) isn't supported by the passage, so we can get rid of it.
(C) says, "unlikely to crave carbohydrates"
From the passage, we know that d-fenfluramine causes an increase in the activity of serotonin in the brain, which results in a reduced craving for carbohydrates (lines 45-49). From this, we know that (C) is correct, as it's very likely that a person who consumes d-fenfluramine will probably not be craving carbohydrates.
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